This section provides background information related to the present disclosure which is not necessarily prior art.
Electric and hybrid vehicles utilize electric motors to provide drive torque to the vehicle wheels. Electric traction motors used as the main drive system for electrified vehicles in some instances are located inside the drive unit and are lubricated and cooled using automatic transmission fluid (ATF). During operation, the permanent magnets used in traction motors are exposed to ATF at relatively high operating temperatures; the motors can reach 150-160° C. during vehicle operation. Thermal aging studies conducted for times as long as 2000 hours have shown that compression molded and injection molded NdFeB magnets exposed to ATF aged more rapidly than the same magnets aged in air, and the degradation of permanent magnetic properties, especially the intrinsic coercivity, can be as large as 18%. Further study identified hydrogen released from the ATF and absorbed by the magnetic material as the most probable cause of the accelerated thermal aging of the magnetic material within the ATF. The present disclosure seeks to reduce or eliminate thermal aging of bonded magnets that are utilized in environments exposed to transmission fluid or similar organic liquids by protecting the magnetic powder from hydrogen exposure.
Currently, full density NdFeB magnets such as sintered magnets are often surface coated with a barrier intended to protect the magnet from corrosion due to oxygen, moisture or salts. In many cases, compression molded magnets are also surface coated for the same reason. The barrier material is not selected on the basis of its low permeability to hydrogen. The binder materials are also not selected to have low permeability to hydrogen.